This invention relates to a cellular type mobile communication system where a mobile station carries out communication with one or more base stations installed in a service area and, in particular, to a method of controlling transmission power in the cellular type mobile communication system where each base station carries out control of the transmission power in accordance with a control command for the transmission power that the mobile station transmits.
As is well known in the art, various multiple access types have been adapted in a mobile communication system. One of the multiple access type is a CDMA (code division multiple access) cellular type. The CDMA cellular type mobile communication system assigns to each channel with a particular code, transmits to the same repeater a modulated wave to which a carrier having the same carrier frequency is spectrum-spread with the code, establishes code synchronization in each receiving side, and identifies a desired channel. The CDMA cellular type mobile communication system may be called a SSMA (spread spectrum multiple access) cellular type mobile radio communication system.
Inasmuch as a plurality of channels use the same frequency in the CDMA cellular type mobile communication system, a received wave power (a desired wave power) of a signal in a channel becomes an interference wave power for other channels. Accordingly, in a reverse link or an upward channel through which the mobile station transmits an upward channel signal and each base station receives the upward channel signal, when the desired wave power is equal to a predetermined reference power or more, a channel capacity decreases because the interference wave power increases. To prevent this, it is necessary to severely control transmission power of the mobile station. The transmission power control for the reverse link is carried out as follows. That is, the base station measures the desired wave power of the upward channel signal and compares the desired wave power with a control target power. When the desired wave power is larger than the control target power, the base station transmits a reverse control command indicating decrement of a reverse transmission power to the mobile station. When the desired wave power is smaller than the control target power, the base station transmits the reverse control command indicating increment of the reverse transmission power to the mobile station. The mobile station decreases or increases the reverse transmission power for the upward channel signal in accordance with the reverse control command. This method of controlling the reverse transmission power is described in U.S. Pat. No. 5,056,109 which is issued to Gilhousen et al. on Oct. 8, 1991 in detail and which has a title of xe2x80x9cMethod and apparatus for controlling transmission power in a CDMA cellular mobile telephone system.xe2x80x9d In order to transmit the reverse control command in this transmission power control, a forward link or a downward channel through which the base station transmits a downward channel signal to the mobile station is used.
On the other hand, a large channel capacity is realized in the forward link by carrying out a forward link transmission power control so that a ratio of the desired wave power to the interference wave power or a signal-to-interference ratio (SIR) has a predetermined value. The transmission power control for the forward link is carried out as follows. That is, the mobile station measures a reception quality for the forward link and compares the reception quality with a control target quality. When the reception quality is higher than the control target quality, the mobile station transmits a forward control command indicating decrement of the forward transmission power to the base station. When the reception quality is lower than the control target quality, the mobile station transmits the forward control command for indicating increment of the forward transmission power to the base station. In addition, the base station increases or decreases the forward transmission power for the downward channel signal in accordance with the forward control command.
However, in the above-mentioned methods, when propagation loss from the mobile station to the base station suddenly increases with moving of the mobile station, the base station may cannot receive the forward control command from the mobile station and the mobile station may cannot receive the reverse control command. Under the circumstances, in a conventional method where the base station controls the forward transmission power in accordance with only the forward control command from the mobile station, the base station does not increase the forward transmission power for the downward channel signal while the base station cannot receive the forward control command from the mobile station when a state where the propagation loss increases continues. In this event, a problem arises that an interruption condition continues in communication between the base station and the mobile station. This is because the mobile station cannot receive the reverse control command from the base station and the mobile station does not increase the reverse transmission power for the upward channel signal of the reverse link.
In addition, generally, a part of user information such as a speech signal or data in a received signal in the base station is encoded with a relatively longer information amount collected so as to correctly decode by carrying out error correction or the like although a received error occurs momentarily. In addition, on decoding, the base station collectively decodes the received signal having the long information amount with a relatively longer time interval. However, in a case where the mobile station moves at a high speed, an error of decision in the forward control command is comparatively much when a high-speed transmission power control is carried out so that the reception quality is maintained at a constant by following variation of a high-speed fading in a propagation path. This is because it is impossible for the decision of the forward control command to obtain the effects of the error correction or the like although the user information may be correctly decoded because it is necessary to momentarily carry out the decision of the forward control command.
Such an error of the decision of the forward control command continuously occurs at a relatively high probability. This is because the error of the decision of the forward control command occurs related to increasing and decreasing of the propagation loss. In addition, when the error of the decision of the forward control command continuously occurs, the base station cannot control the forward transmission power for the downward channel signal in accordance with the forward control command and the mobile station may be put into a state where the mobile station does not correctly receive the downward channel signal. On the other hand, inasmuch as the mobile station cannot receive the reverse control command included in the downward channel signal from the base station, it may be impossible to control the reverse transmission power for the upward channel signal. Under the circumstances, the error of the decision of the forward control command in the upward channel signal occurs frequently and it may be impossible for the base station to correctly receive the user information. In such a case, a problem occurs that the interruption condition continues in the communication between the base station and the mobile station.
In addition, in the cellular type mobile communication system, there is a technique called a soft handover or a soft handoff where a mobile station simultaneously establishes channels with a plurality of base stations at the vicinity of a boundary in cells when the mobile station moves between the cells and changes the channels between the cells. This technique is especially an important technique in the cellular type mobile communication system which adapts the CDMA as the multiple access. Such a soft handover is described in U.S. Pat. No. 5,101,501 which is issued to Gilhousen et al. on Mar. 31, 1992 in detail and which has a title of xe2x80x9cMethod and system for providing a soft handoff in communications in a CDMA cellular telephone system.xe2x80x9d
In a case of carrying out the transmission power control for the reverse link or the upward channel during execution of such a soft handover, a plurality of base stations measure the desired wave power of the upward channel signal transmitted from the mobile station and transmit reverse control commands for the reverse transmission power to the mobile station, independently. In addition, the mobile station receives the reverse control commands and preferentially complies with one of the reverse control commands that is indicative of lowering the reverse transmission power when the mobile station receives the reverse control commands which are different from one another. This method is disclosed in xe2x80x9cTIA/EIA INTERRIM STANDARD, Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95-A (Revision of TIA/EIA/IS-95)xe2x80x9d, Telecommunications Industry Association, May 1995, in a section of xe2x80x9c6.6.6.2.7.2 Reverse Traffic Channel Power Control During Soft Handoff.xe2x80x9d As described in the above-reference, by preferentially complying with the reverse control command indicative of lowering the reverse transmission power when the reverse control commands are different from one another among the base stations, it is possible to prevent that the desired wave power of the upward channel signal exceeds the control target power in any base station and results in realizing a high channel capacity in the reverse link or the upward channel. Accordingly, it is important for this method that the mobile station can receive the reverse control commands from all of the base stations which may have the minimum propagation loss for the reverse link or the upward channel.
It is important for the transmission power control for the forward link or the downward channel during execution of the soft handover to carry out so that the mobile station can receive the reverse control commands from all off the base stations which may have the minimum propagation loss for the reverse link or the upward channel.
For this purpose, it may hit one skilled in the art upon a method for controlling the forward transmission power on the forward link or the downward channel so that the desired wave powers of the downward channel signals transmitted from the base stations are equal to one another in the mobile station. However, this method is disadvantageous in that the interference wave power increases and a capacity of the forward link decreases. This is because the base station having the large propagation loss to the mobile station sets a large forward transmission power by its part. As a method for restraining the capacity of the forward link decreasing, there is a method for controlling so that the forward transmission powers of the downward channel signals transmitted from the base stations are equal to one another. This method is described in an article which is contributed by Andersson to Proc. IEEE 44th Vehiclar Technology Conference, (June 1994), pages 41-45, and which has a title of xe2x80x9cTuning the macro diversity performance in a DS-CDMA system.xe2x80x9d
In this method, when a received power of the reverse control command transmitted from a base station having a small propagation loss to the mobile station is larger than another received power of the reverse control command transmitted from another base station having a large propagation loss and when a difference therebetween is large, there is a high probability that the mobile station fails in receiving the reverse control command from the other base station having the large propagation loss. However, this case is not normally a problem. This is because the reverse transmission power of the reverse link is mainly controlled by the base station having the small propagation loss. On the other hand, when a difference between the propagation losses is small, it is important that the reverse transmission power is controlled in accordance with both of the base stations. Under the circumstances, there is a high probability that it is possible to correctly receive both of the reverse control commands. This is because the mobile station can receive the reverse control commands at nearly equal powers. Accordingly, in order to control the reverse transmission power of the reverse link, the mobile station can receive all of the reverse control commands from the base stations where the propagation loss of the reverse link may become minimum.
In addition, during execution of the soft handover, in a case where the magnitude of the propagation loss from the mobile station to each of the base stations rapidly changes, the base station having the minimum propagation loss carries out transmission at any moment although the base stations carrying out transmission to the mobile station are changed at a high speed in accordance with it. In this event, if the transmission powers of the base stations are not equal to one another, the reception quality may be easily degraded when the base station having the minimum propagation loss changes. This is because the reception quality increases or decreases. However, if the transmission powers of the base stations are equal to one another, it is possible to improve the reception quality more due to a diversity effect where the reception quality is maintained at a constant nearly although the base station having the minimum propagation loss changes.
With the transmission power control for the forward link or the downward channel as described above, the mobile station measures the reception quality of the forward link, compares the reception quality with a control target quality, transmits the forward control command indicating decrement of the forward transmission power to the base stations if the reception quality is higher than the control target quality, and transmits the forward control command indicating decrement of the forward transmission power to the base stations if the reception quality is lower than the control target quality. During execution of the soft handover, a plurality of base stations receive the forward control command which the mobile station transmits. In addition, each of the base stations controls the forward transmission power for the upward channel signal so as to increase or decrease the forward transmission power in accordance with the forward control command. Accordingly, inasmuch as increment or decrement of the forward transmission powers is repeated in the base stations in the similar manner if the base stations have initial values of the forward transmission power that are equal to one another, the forward transmission powers are controlled in the base stations so that the forward transmission powers are equal to one another if each of the base stations receives the forward control command with no error.
However, this method is disadvantageous in that the base station having the large propagation loss frequently fails to receive the forward control command from the mobile station because received power of the forward control command is small although the base station having the minimum propagation loss for the mobile station can almost correctly receive the forward control command from the mobile station. Accordingly, it is difficult to maintain so that the forward transmission powers of the base stations are equal to one another.
Under the circumstances, it will be assumed that the forward transmission power for the base station having the large propagation loss for the mobile station is smaller than the forward transmission power for the base station having the minimum propagation loss for the mobile station caused by failure of reception of the forward control command. In this event, an error of reception may easily occur in the reverse control command for transmission power control of reverse link (the upward channel) that the base station having the large propagation loss for the mobile station transmits. As described above, inasmuch as it is important in the transmission power control for the reverse link that the mobile station can receive the reverse control commands from all of the base stations which may have the minimum propagation loss for the reverse link, such an error of the reception of the reverse control command results in decreasing of a channel capacity for the reverse link. In addition, the reception quality may be easily degraded by decreasing of the above-mentioned diversity effect where a plurality of base stations transmit at the powers which are equal to one another.
On the other hand, it will presumed that the forward transmission power for the base station having the large propagation loss for the mobile station is larger than the forward transmission power for the base station having the minimum propagation loss for the mobile station caused by failure of reception of the forward control command. In this event, problems where interference wave power increases and capacity of the forward link decreases arise. This is because the base station having the large propagation loss for the mobile station sets the forward transmission power large in the manner in a case of the above-mentioned method of controlling the transmission powers of the forward link so that the desired wave powers from the base stations are equal in the mobile station to one another.
Measures are taken that the base station having the large propagation loss for the mobile station fails in receiving of the forward control command from the mobile station due to the large propagation loss. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 9-312,609 or JP-A 9-312,609 discloses a method of independently controlling the forward transmission power of the forward link in each base station in defiance of the forward control command which has a low reception quality and which has less reliability. However, in this method, the base stations do not transmit at the transmission powers which are equal to one another and it is impossible in this method to resolve the above-mentioned problems.
In addition, JP-A 9-312,609 also describes a method of the steps of gathering, in a combining station (a control station), control signals from a mobile station via base stations to produce a gathered control signal and of controlling, in response to the gathered control signal, a forward transmission power for a forward link in each base station. On realizing a high-speed transmission power control according to this method, this method is disadvantageous in that an amount of transmission for the control signals increases between each of the base station and the control station. On the other hand, if the amount of transmission for the control signals is restricted, this method is advantageous in that it has a large delay and it is difficult to realize the high-speed transmission power control.
It is therefore an object of the present invention to provide a transmission power control method for a cellular type mobile communication system that is capable of continuously carrying out communication without continuing an interruption state of communication between a base station and a mobile station in a case where a propagation loss rapidly increases between the base station and the mobile station.
It is another object of the present invention to provide a transmission power control method for a cellular type mobile communication system that is capable of continuously carrying out communication without continuing an interruption state of communication between a base station and a mobile station in a case where an error continuously occurs in decision of a forward control command transmitted from the mobile station in the base station.
It is still another object of the present invention to provide a transmission power control method for a cellular type mobile communication system in which a plurality of base stations can carry out transmission at powers which are nearly equal to one another although a reception error occurs in a forward control command for a transmission power control of a forward link transmitted from a mobile station in a base station during execution of a soft handover without excessively increasing an amount of transmission of control signals between the base station and a control station.
It is yet another object of the present invention to provide a transmission power control method for a cellular type mobile communication system of the type described, that is capable of obtaining a high capacity in both a reverse link and a forward link.
Other objects of this invention will become clear as the description proceeds.
According to this invention, while a mobile station establishes a channel with one or more base stations, each base station repeatedly carries out a transmission power control operation for, on reception of a control command from the mobile station, increasing or decreasing a transmission power in response to the control command and for renewing the transmission power so that the transmission power of the base station after increased or decreased approaches a predetermined reference power.
For this end, while the mobile station establishes the channel with one or more base stations, each base station increases or decreases, on reception of the control command from the mobile station, the transmission power in response to the control command and renews the transmission power so that a difference (Paxe2x88x92C) between the transmission power Pa of the base station after increased or decreased and a predetermined transmission power C is r times as large as a difference (Pbxe2x88x92C) between the transmission power Pb before renewed and the predetermined reference power C, namely,
Pa=r(Pbxe2x88x92C)+C
where r represents a coefficient which is not less than zero and which is less than one, namely, (0xe2x89xa6r less than 1).
A value of the coefficient r is determined so that a product (Pmaxxe2x88x92Pmin) (1xe2x88x92r) of a difference (Pmaxxe2x88x92Pmin) between a maximum value Pmax and a minimum value Pmin in a control range for the transmission power P and another coefficient (1xe2x88x92r) is less than a changing step xcex94P for the transmission power in a case of increasing or decreasing the transmission power in response to the control command, namely,
(Pmaxxe2x88x92Pmin)(1xe2x88x92r) less than xcex94P.
Specifically, on describing the gist of an aspect of this invention, it is possible to be understood that a method is of controlling transmission power in a cellular type mobile communication system comprising a plurality of cells, a plurality of base stations disposed in the respective cells, and a mobile station located in the cells. The mobile station carries out communication with at least one base stations by establishing a channel between the mobile station and the at least one base station. According to the aspect of this invention, the above-understood method comprises the steps of repeatedly carrying out a first transmission power control operation where the mobile station transmits a control command for the transmission power to the at least one base station while the mobile station establishes the channel with the at least one base station and the at least one base station receives the control command to change the transmission power in response to the control command, and of repeatedly carrying out a second transmission power control operation which renews the transmission power in the at least one base station so that an absolute value of a difference between a predetermined reference power in dB and the transmission power in dB decreases at a predetermined rate.
In addition, in the method of controlling the transmission power in the cellular type mobile communication system, the predetermined reference power may be equal to a maximum transmission power.
Furthermore, according to this invention, while a mobile station establishes channels between the mobile station and a plurality of base stations, the base station repeats a transmission control operation for increasing or decreasing, on reception of a control command for transmission power from the mobile station, the transmission power in response to the control command, and for renewing the transmission power so that the transmission powers in the plurality of base stations after increased or decreased have less difference and approach a reference power which is defined in the plurality of base stations in common.
For this purpose, while the mobile station establishes the channels with the plurality of base stations, each base station increases or decreases, on reception of the control command from the mobile station, the transmission power in response to the control command and renews the transmission power so that a difference (Paxe2x88x92C) between the transmission power Pa of the base station after increased or decreased and a predetermined transmission power C defined in the plurality of base stations in common is r times as large as a difference (Pbxe2x88x92C) between the transmission power Pb before renewed and the predetermined reference power C, namely,
Pa=r(Pbxe2x88x92C)+C,
where r represents a coefficient which is not less than zero and which is less than one, namely, (0xe2x89xa6r less than 1).
In addition, in the method of controlling the transmission power in the cellular type mobile communication system according to this invention, the coefficient r is determined so that a product (Pmaxxe2x88x92Pmin)(1xe2x88x92r) of a difference (Pmaxxe2x88x92Pmin) between a maximum value Pmax and a minimum value Pmin in a control range for the transmission power P and another coefficient (1xe2x88x92r) is less than a changing step xcex94P for the transmission power in a case of increasing or decreasing the transmission power in response to the control command, namely,
(Pmaxxe2x88x92Pmin)(1xe2x88x92r) less than xcex94P.
Specifically, on describing the gist of an aspect of this invention, it is possible to be understood that a method is of controlling transmission power in a cellular type mobile communication system comprising a plurality of cells, a plurality of base stations disposed in the respective cells, and a mobile station located in the cells. The mobile station carries out communication with one or more base stations by establishing one or more channels between the mobile station and the one or more base stations. According to the aspect of this invention, the above-understood method comprises the steps of repeatedly carrying out a first transmission power control operation where the mobile station transmits a control command for the transmission power to each of the plurality of base stations while the mobile station establishes the channels with the plurality of base stations and each of the plurality of base stations receives the control command to change the transmission power in response to the control command, and of repeatedly carrying out a second transmission power control operation which renews the transmission power in each of the plurality of base stations so that an absolute value of a difference between a reference power in dB defined in the plurality of base stations in common and the transmission power in dB decreases at a common rate among the plurality of base stations.
In addition, in the method of controlling the transmission power in the cellular type mobile communication system, the reference power may be equal to a maximum transmission power.
Furthermore, in the method of controlling the transmission power in the cellular type mobile communication system, the reference power may be equal to an intermediate power between a maximum transmission power in dB and a minimum transmission power in dB.
In addition, in the method of controlling the transmission power in the cellular type mobile communication system, the reference power may be equal to a minimum transmission power.
In addition, in the method of controlling the transmission power in the cellular type mobile communication system, the plurality of base stations calculate statistical values of the transmission power to transmit the statistical values to a control station. The control station calculates a reference value indicating the reference power using the statistical values of the plurality of base stations to send the reference value to the plurality of base stations. Each of the plurality of the base stations uses the reference power having the reference value sent from the control station.
Furthermore, in the method of controlling the transmission power in the cellular type mobile communication system, the control station may use, as the reference value, a maximum one of the statistical values.
In addition, in the method of controlling the transmission power in the cellular type mobile communication system, the control station may determine the common rate in accordance with mutual differences among the statistical values of the plurality of base stations.
Furthermore, in the method of controlling the transmission power in the cellular type mobile communication system, the control station may determine frequency for renewing the transmission power by the common rate in accordance with mutual differences among the statistical values of the plurality of base stations.
In addition, in the method of controlling the transmission power in the cellular type mobile communication system, the mobile station may have an accumulated control value for the transmission power in the base stations. In this event, the mobile station renews the accumulated control value on transmitting the control command for the transmission power to the base station. The mobile station transmits the accumulated control value to the plurality of base stations at a time interval which is longer than a repetition time interval of a changing operation for the transmission power. Each of the plurality of base stations determines the reference power using the accumulated control value received therein to use the reference power.
Furthermore, in the method of controlling the transmission power in the cellular type mobile communication system, the plurality of base stations may simultaneously carry out a renewal of the respective transmission powers.
A base station in a cellular type mobile communication system according to this invention repeatedly carries out, while a mobile station establishes a channel with one or more base stations, a transmission power control operation for, on reception of a control command from the mobile station, increasing or decreasing a transmission power in response to the control command and for renewing the transmission power so that the transmission power of the base station after increased or decreased approaches a predetermined reference power.
For this end, while the mobile station establishes the channel with one or more base stations, the base station increases or decreases, on reception of the control command from the mobile station, the transmission power in response to the control command and renews the transmission power P so that a difference (Paxe2x88x92C) between the transmission power Pa of the base station after increased or decreased and a predetermined transmission power C is r times as large as a difference (Pbxe2x88x92C) between the transmission power Pb before renewed and the predetermined reference power C, namely,
Pa=r(Pbxe2x88x92C)+C,
where r represents a coefficient which is not less than zero and which is less than one, namely, (0xe2x89xa6r less than 1).
The coefficient r may have a value determined so that a product (Pmaxxe2x88x92Pmin)(1xe2x88x92r) of a difference (Pmaxxe2x88x92Pmin) between a maximum value Pmax and a minimum value Pmin in a control range for the transmission power P and another coefficient (1xe2x88x92r) is less than a changing step xcex94P for the transmission power in a case of increasing or decreasing the transmission power in response to the control command, namely
(Pmaxxe2x88x92Pmin)(1xe2x88x92r) less than xcex94P.
Specifically, on describing the gist of an aspect of this invention, it is possible to be understood that a base station is for use in a cellular type mobile communication system comprising a plurality of cells, a plurality of base stations disposed in the respective cells, and a mobile station located in the cells. The mobile station carries out communication with at least one base stations by establishing a channel between the mobile station and the at least one base station. According to the aspect of this invention, the above-understood base station comprises a first transmission power controller for repeatedly carrying out a first transmission power control operation for changing the transmission power in response to a control command for, the transmission power that is received from the mobile station while the mobile station establishes the channel with the at least one base station, and a second transmission power controller for repeatedly carrying out a second transmission power control operation for renewing the transmission power so that an absolute value of a difference between a predetermined reference power in dB and the transmission power in dB decreases at a predetermined rate.
In addition, in the base station in the cellular type mobile communication system, the predetermined reference power may equal to a maximum transmission power.